Kelsey Schroeder was "born green," according to her mother Angela, and she takes that environmental enthusiasm to class with her at the Oak Knoll School of the Holy Child in Summit, N.J. The 12-year-old has been a driving force in greening her school since she was a fourth-grader, pushing teachers and classmates alike to cut waste and use less energy. But what really motivates kids--especially the sort of achievers who attend an exemplary private school like Oak Knoll--is a little competition. So when Schroeder and her classmates found out about a website launched last year that pits teams from around the country against one another in a contest to see who could be greener, they jumped on board. Her seventh-grade Royal Acorns team is Carbonrally.com's reigning champ, having saved 11.21 tons of climate-changing CO2 to date.

As Americans grow more green-minded, more of them want to approach environmentalism in concrete terms. Thanks to websites like Carbonrally, one increasingly popular way to do so is by measuring and measurably reducing our carbon footprints--the greenhouse gases we're responsible for emitting. The more dependent we are on fossil fuels, the bigger our carbon footprints; unsurprisingly, Americans, who are responsible for more than 20 tons of CO2 per capita annually, have some of the biggest feet in the world. How big? A recent study by a class at the Massachusetts Institute of Technology found that even a homeless American would have a carbon footprint of 8.5 tons--twice the global average. "We have contributed more than our fair share to this problem," says Katherine Wroth, a senior editor at the green website Grist.org "It seems logical that we would want to contribute to the solution."

Especially if contributing to a solution feels like playing a competitive sport. Carbonrally lays out environmental challenges and keeps score by translating green actions into pounds of carbon dioxide averted. For instance, cutting your daily shower time by two minutes for a month--a recent challenge--reduces CO2 emissions by 15.3 lbs. "This has been a great motivation technique," says Schroeder, who has logged individual savings of more than 1,000 lbs. of CO2 on Carbonrally. "We just want to keep going and see if we can do better."

Learning your approximate carbon shoe size is the first step. Everything you do that is powered by fossil fuels has a carbon dioxide cost, and it adds up--a bit like credit card debt. Some actions, like commuting in a gasoline-powered car, have obvious carbon costs. Others are less clear but still significant. Take your diet: livestock are responsible for an estimated 18% of global carbon emissions, so when you chow down a hamburger, you're effectively emitting CO2 as well. Even something as small as an iPod Nano will add to your carbon footprint, thanks to both the energy used to produce and ship it and the energy later needed to charge it (68 lbs. of CO2 over its lifetime, according to the British design consultancy IDC).

Of course, carbon is such a universal ingredient of modern life that it's impossible to measure exactly how big your individual footprint is. But you can get a decent estimate at a number of websites. One of the best is run by the Nature Conservancy, which leads you through a detailed questionnaire on your home energy use, driving, flying and diet. The Environmental Protection Agency (EPA) has a carbon calculator that not only sizes up your footprint but also allows you to see how changing your behavior--like driving less--can reduce your impact. No two carbon calculators are the same, since footprinting is still an inexact science. But using one from a green group or a government agency--and not one sponsored by a company--should give you an unbiased number that will help identify exactly where you can go greener. "Global warming is an abstract idea that is hard for people to connect to," says Bob Schildgen, the Sierra Club's environmental-advice columnist. "It's good to start at the basic level, with real numbers."

Carbonrally's challenges offer a useful start. A recent contest involved giving up meat for two days, which would reduce carbon emissions by 13.2 lbs. Another called on competitors to unplug their computers every night for one month, which cuts CO2 by 51 lbs. Each contest illustrates a basic way to shrink your footprint: remove meat from your diet, drive or fly less and just reduce the amount of power you use at home whenever possible, either through conservation or with appliances that are more energy-efficient. Winners get small prizes like tote bags--and green bragging rights. (Those who want to go the extra green mile can purchase offsets, which purport to balance out your emissions by funding a carbon-reduction project elsewhere in the world.) Altogether, Carbonrally's 2,000 users have averted more than 150 tons of CO2 emissions since the site was launched in October 2007. "We put the challenges in bite-size chunks, but that adds up," says founder Jason Karas.

Given that global CO2 emissions total more than 28 billion tons a year, however, that still doesn't add up to a whole lot. Indeed, since carbon emissions will continue to rise in the developing world no matter what we do, it's worth asking why we should even bother to change our lifestyles. One reason is to show others how it can be done. "None of us believes this will end climate change," says Annabelle Gurwitch, who hosts a show on footprinting called Wa$ted! on the new network Planet Green. "But it lets people feel effective."

That feeling means something. Patricia Palermo, a teacher and green guru at Oak Knoll, points out that her students have been hearing about the coming catastrophe of global warming since they were born. But competing in Carbonrally is contagious--Oak Knoll now has a second team--and it turns the students from passive victims into climate warriors. Fittingly, the Royal Acorns' motto--taken from their school--is "Action, not words." It's a rallying cry more Americans are starting to heed.